For the control of electric motors with several phase windings to which an electric current is supplied alternately and/or with a time overlap, it is necessary that, for example, the currents flowing through the individual phase windings are measured as at least one operating variable. Different measuring devices are known from the prior art, which have a number of current measuring devices S1 to S3 corresponding to the number of phase windings n1 to n3 for measuring associated individual currents i1 to i3, as is shown schematically in FIG. 1 for the prior art. For reasons of symmetry, a number of current measuring devices (=n−1) is used, which is one less than the number of phase windings, wherein the missing partial current can easily be determined based on the other measured partial currents. Because of the required number of individual current measuring devices and the high costs of the individual current measuring devices, a device of this kind for measuring current is very elaborate and expensive.
Furthermore, it is disclosed in DE 102 51 158 A1 to measure the total current of all individual currents by means of a single current measuring device and subsequently to calculate the electrical currents in the individual phase windings by means of an electronic unit based on a model for the drive unit. Estimates of this kind based on models can lead to errors in calculating the individual currents. These errors are particularly large at certain frequencies and loads of the electric motor.